1. Field of the Invention
This invention relates to puffer-type circuit interrupters and more particularly to puffer-type circuit interrupters having a pair of separable contacts disposed within a casing containing an arc extinguishing fluid and arranged to extinguish the electric arc with a puff of the arc extinguishing gas highly pressurized by the energy of the arc established upon the separation of the contacts.
2. Description of the Prior Art
In a circuit interrupter using an arc extinguishing fluid, it is an effective measure for improving the arc extinguishing capability of the interrupter to puff an arc extinguishing fluid to an electric arc to diffuse and cool the arc. In order to obtain a strong puff of the arc extinguishing gas, it is necessary to provide means for establishing a substantial pressure difference between both ends of the arcing region. The typical circuit interrupter of this type includes a puffer-type circuit interrupter wherein the pressure difference is obtained by a puffer structure operable in response to the contact opening operation and a double pressure type wherein a separate high pressure source driven by a compressor supplies the necessary high pressure gas through a valve mechanism operable in response to the contact opening operation.
However, since the puffer-type circuit interrupter must operate the puffer mechanism by a mechanical means in response to the contact opening operation, the operating force must be large. This operating force increases with the increase in the capacity of the interrupter because the puffer device increases its size as the choking arc current of the electric arc increases. With the larger operating mechanism, the mechanical strength requirement of the mechanical linkage for transmitting the operating force becomes more severe. Further, since the puffer load is very small in the unloaded or small current opening operation which is most of the contact opening operation, when the interrupter is operated under such conditions a large excessive operating force is generated due to the mechanism designed to provide a massive operating force necessary for high current interruption. This excessive force accelerates the contact at an abnormal rate to such an extent that current shearing and the generation of abnormal voltage is observed, leaving many difficult problems from practical and economical points of view.
The double pressure type circuit interrupter requires double pressure systems, and many associated devices such as valves and compressors and controls therefor, making the overall structure of the interrupter large-sized and complicated.
It has recently been proposed to utilize the pressure-raising function of the electric arc mainly due to the thermal energy of the arc itself to provide a high pressure gas, which gas is released through the arcing region to the established electric arc during the decrease of the arc current to the zero value, thereby extinguishing the arc. With this type of interrupter, since the high pressure gas is formed mainly owing to the thermal energy of the arc, the generated high pressure gas is naturally also at a high temperature. When the arc extinguishing gas is hot, it has a low density, promotes ionization, and is of deteriorated insulating capability and decreased arc diffusion and cooling function, thereby rendering its arc extinguishing function substantially degraded. This phenomenon is aggravated when an attempt is made to improve the performance by increasing the pressure-raising effect to generate a higher pressure because the higher pressure is always accompanied by a higher temperature which makes the gas have a higher conductivity, thus degrading the arc extinguishing capability of the arc extinguishing gas. This makes the interrupter disadvantageous in that it has a limit to improving the arc extinguishing performance so that a large-capacity circuit interrupter is very difficult to obtain.